1. Field of the Invention
The present invention relates to image signal processing techniques, and in particular relates to image signal processing techniques for image enhancement.
2. Description of the Related Art
A Bayer filter, commonly used in a digital camera or a camcorder, is a color filter array (CFA) for arranging RGB color filters on a square grid of an image sensor. FIG. 1 shows an arrangement pattern of the Bayer filter. Since human eyes are most sensitive to green light, the Bayer filter uses twice as many green elements as red or blue in the image sensors. As can be seen from FIG. 1A, the arrangement pattern of the Bayer filter is 50% green, 25% red and 25% blue.
FIG. 2 illustrates the processes of the image signal processing (ISP) technique in the prior art. After the color sensing procedure (step S202) is performed on the image sensor, the demosaicking procedure (step S204) proceeds to use a form of interpolation to reconstruct a full color image from the color data (in raw format) obtained by the image sensor. In the full color image, each of the pixels has an RGB signal including a red (R), a green (G), and a blue (B) component. In the prior art, in order to perform image enhancement such as noise reduction or sharpness enhancement (step S208), the RGB signals of the pixels have to be converted, by the so-called color space conversion (CSC) procedure (step S206), to luminance and chrominance signals (i.e., YCbCr signals) since, in the prior art, luminance is regarded as being more important in the procedures of reducing noise or enhancing sharpness of an image than color (thus, the image enhancement procedure in the prior art is usually performed only with the luminance signal rather than the chrominance signal). At last, after image enhancement (step S208), the enhanced luminance and chrominance signals will be further converted back to the RGB signals (step S210).
Based on the same reason for using the Bayer filter (that is, human eyes is most sensitive to green light), the green sensor (sensor covered by green color filter) is usually designed to have better and higher signal to noise ratio (SNR) than the other two color sensors, which are the red and blue sensors. As a result, the prior art using the luminance signal, which is converted from the RGB signal and much more influenced by the green color signal than the other two color signals (for example, according to ITU 601 standard, the relationship between the luminance signal Y and the RGB signal is: Y=16+0.299×R+0.587×G+0.114×B), for image enhancement, may introduce more noise from the luminance signal and finally produce a processed image with more noise after image enhancement. Moreover, doing sharpness enhancement on luminance only may make false color phenomenon caused by the demosaicking procedure become even worse.
Therefore, it is desirable to provide a new image signal processor or method which can overcome the above issues.